Controlling combustion often has been likened to a black art rather than a science. The physics of burning coal are as well understood as the design of steam generators. The art of optimizing combustion to produce minimal emissions comes into play when a boiler is asked to burn a fuel for which it was not designed and yet produce orders of magnitude less pollution than original permitted levels. We shouldn't be surprised when a new round of boiler capital improvements is required every few years to comply with a new, lower NOx limit.
For many coal-fired plants, the recipe for optimized combustion has three ingredients: common sense, skepticism, and a willingness to lower expectations. Realize that maximizing the performance of every plant system isn't sufficient to maximize the performance of the overall plant. A more holistic approach is required. The contributions of all systems to overall plant performance must be quantified and addressed separately and in concert. Permanently solving a combustion problem requires thoroughly understanding its underlying mechanisms, targeting them with good engineering practices, and having the patience to try different solutions until the desired results are achieved.
The antithesis of this approach would be succumbing to the temptation to look for the easy way out. Be skeptical of vendors' claims. Don't expect to solve your combustion problem overnight by buying the latest widget or software package and expecting your already-overburdened O&M staff to find a way to make it work.
About the patient and project
The 126-MW AES Westover Station in Johnson City, N.Y., provides an excellent case study of how applying the fundamentals and taking a holistic approach to a combustion problem can yield exemplary results. Westover Station Unit 8 is a tangentially-fired unit manufactured by Combustion Engineering (now Alstom Power) whose original steam conditions were 620,000 lb/hr of main steam at 1,475 psig. Superheat and reheat temperatures are 1,005F. The unit now has a gross output of 88 MW. The steam generator's furnace measures 24 ft, 10 in. deep by 25 ft, 4 in. wide. The boiler's 16 burners at four elevations are fed by four Raymond No. 533 deep-bowl pulverizers.
When Unit 8 entered commercial service in 1951, its NOx emission rate was typical of the time—0.6 to 0.9 lb/mmBtu with 3% loss-on-ignition (LOI). Subsequent combustion modifications reduced the rate to about 0.5 lb/mmBtu but raised LOI to 20%. Plant management was asked to modify Boiler 13 as needed to lower its NOx output below 0.32 lb/mmBtu as part of an AES fleetwide emissions reduction plan. The other goals of the project were to:
- Lower Unit 8's NOx emission rate from over 0.5 lb/mmBtu (at full load) to 0.32 lb/mmBtu or less.
- Reduce LOI (flyash carbon content) to 5% to 8% (or less) at 83-MW net load .
- Maintain the excess-oxygen concentration at the furnace exit at 2.0% minimum (by traversing a test grid with a high-velocity thermocouple probe to create a "slag-friendly" furnace).
- Maximize the unit's load response and fuel flexibility.
- Optimize its overall combustion efficiency and performance.
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